#include<iostream>
#include<cmath>
using namespace std;
int main()
{
int a,b,c;
scanf("%d%d%d",&a,&b,&c);
if(a<=0 || b<=0 || c<=0)
{
printf("no");
}
if(a+b>c && a+c>b && b+c>a) 
{
float p = 1.0*(a+b+c)/2;
float s = sqrt(p*(p-a)*(p-b)*(p-c));
printf("S= %f",s);
}
else
{
printf("no");
}
}
